1. Field of the Invention
The present invention relates to power dividers and combiners. More specifically, the present invention relates to high power, large bandwidth power divider and combiners operating at S-band for driving, C-class amplifiers.
2. Description of the Related Art
Power dividers are used to direct microwave radio frequency (RF) power from one source to two or more outputs. Likewise, power combiners are used to combine power from two or more sources to provide one output. Currently, the most widely used coupler in the RF field is the overlay hybrid four port Coupler. The overlay hybrid four port coupler is a two-way combiner having two inputs, each of which communicates with two outputs. It is an xe2x80x98overlayxe2x80x99 coupler because one or more of the conductors thereof couple energy to an adjacent conductor by physically overlaying the adjacent conductor. The overlay hybrid four port coupler is widely used because it improves the voltage standing wave ratio (VSWR) of a signal notwithstanding, the fact that the inputs may be highly mismatched. N-way combiners are not known to provide such SWR performance.
In binary power dividers and combiners, the inputs are related to the outputs by a power of two. Efficient power combining architectures in high power (e.g. 1 kilowatt (kW) and higher) microwave transmitters require non-binary combining techniques due to the increased power gain of the presently available microwave C-class transistors. (As is well known in the art, microwave C-class bipolar transistors have evolved in the last 6 years from a gain of 7 dB to more than 8.5 dB presently)
For a new RADAR transmitter and other applications, a need has been recognized in the art for a 3:1 hybrid divider/combiner for a non-binary combined 1 kW microwave unit amplifier. Ideally, this coupler will duplicate the performance of the classic 2:1 overlay 90xc2x0 hybrid coupler and should be easy to integrate in the amplifier""s layout. To be used as a combiner in C-class amplifiers. the coupler should have low loss, sufficient bandwidth and good active return loss. It should also be matched at the second harmonic of the operating frequency. When used as a divider, for driving class C transistors, the amplitude imbalance between the output ports of the coupler should be kept to a minimum.
The only known three-way 90xc2x0 coupler that has hybrid properties and can be implemented in a microstrip layout is the N-way branch coupler. This coupler is a generalized form of the classic two-way branch hybrid coupler. (See xe2x80x9cMulti-port Lattice-type Hybrid Networkxe2x80x9d, by Takai Kuroda, Takeshi Usui, and Kazuo Yano, IEEE-GMTT International Microwave Symposium (1971) and xe2x80x9cPlanar Electrically Symmetric N-Way Hybrid Power Dividers/Combinersxe2x80x9d, by A. A. M. Saleh, IEEE Trans on MTT., vol. MTT-28 (June 1980).) This N-way branch coupler can offer good return loss and isolation over a few percent relative bandwidth only. It is highly reactive at the second harmonic of the operating frequency and has poor active return loss, which is not acceptable for C-class amplifiers.
The Wilkinson three-way divider/combiner has limited power performance and requires a tri-dimensional resistive balancing circuit. That is, the Wilkinson divider/combiner has no equal mismatch canceling, modest isolation, no high power capability and is non-planar (3D). In addition, the fringe fields around the balancing resistors increase the insertion loss. (See xe2x80x9cAn N-Way Hybrid Power Dividerxe2x80x9d, E. J. Wilkinson, IRE Trans on MTT., vol. MTT-8 (January 1960).)
Good power capability and isolation are offered by the Gysel combiner, but this design does not have the important property of identical mismatch canceling. In addition. the Gysel combiner is mismatched at the second harmonic. Its complex design requires a large printed wiring board (PWB) area. The result is a large, lossy device in microstrip, which is difficult to implement in planar artwork. (See xe2x80x9cA New N-Way Power Divider/Combiner Suitable For High Power Applicationsxe2x80x9d, U. H. Gysel, 1975 MTT-S International Microwave Symposium.)
The chain combiner can offer good performance but the 3 dB and 4.77 dB overlay couplers required, and the registration requirements thereof, limit the peak power capability and can not be integrated in a microstrip layout. It would also require a special substrate thickness which would be unacceptable in the high power microwave application. (See xe2x80x9cA Microwave Power Dividerxe2x80x9d, by R. J. Mohr, IEEE Trans on MTT. (November 1961) and xe2x80x9cAdrenaline Couplersxe2x80x9d, ANAREN RF and Microwave Components Catalog. 
The star divider has not proven to be feasible for this application. Various other (star divider derived) planar geometries have been explored to create a 1:N divider/combiner but the performances obtained are not satisfactory when compared to what is necessary in a microwave C-class amplifier. (See xe2x80x9cAnalysis and Design of Four-Port and Five-Port Microstrip Disc Circuitsxe2x80x9d, by K. C. Gupta and M. D. Abouzahra, IEEE Trans on MTT., vol. MTT-33, (December 1985); xe2x80x9cMultiple-polt Power Divider/Combiner Circuits Using Circular Microstrip Disk Configurationsxe2x80x9d, by M. D. Abouzahra and K. C. Gupta, IEEE Trans on MTT., vol. MTT-35 (December 1987); and xe2x80x9cMultiport Power Divider-Combiner Circuits Using Circular-Sector-Shaped Planar Componentsxe2x80x9d, M. D. Abouzahra and K. C. Gupta, IEEE Trans on MTT., vol. MTT-36,(December 1988).)
There is a need for a Coupler that is broadband, provides good active return loss and can be implemented on a soft substrate ∈rxe2x89xa63. Ideally, the coupler would have a 3:1 combining/division ratio and should retain all hybrid advantages and properties. There is a further need for a design that is easily manufactured using standard procedures and offers a sufficiently low imbalance and VSWR under expected manufacturing tolerances to be adequate for C-class amplifiers. This will make possible the implementation of the 3:1 coupler directly into the amplifier""s layout.
The need in the art is addressed by the power divider and combiner of the present invention. The inventive divider/combiner includes first, second and third weakly coupled transmission lines. The first transmission line provides first and second ports at first and second ends thereof, respectively. The second transmission line provides third and fourth ports at first and second ends thereof, respectively and the third transmission line provides fifth and sixth ports at first and second ends thereof, respectively.
In the illustrative embodiment, the first, second and third transmission lines are coupled to provide equal outputs at said second, fourth and sixth ports in response to an application of a signal at the first port.
The inventive divider/combiner may be implemented with coaxial, stripline or microstrip type transmission lines. The looser coupling of the present invention is very beneficial, especially in microstrip, to obtain high power capability and a manufacturable circuit. In the illustrative 3:1 divider/combiner implementation, the coupling arrangement provides a voltage coupling coefficient x equal to 0.325057. Consequently, the first, second and third coupling lines have a relative coupling value of approximately xe2x88x9210 decibels. In the best mode, the first, second and third coupling lines have a relative coupling of xe2x88x929.76 decibels.